The invention relates to multi-layer printed circuit boards and a method for determining the actual position of internally disposed terminal areas on the individual conductive layers of pressed together sheet-like board sections forming the multi-layer printed circuit boards.
One of the most important work steps in the manufacture of multi-layer printed circuit boards is the drilling of the terminal holes for the metallic through-connections. In order to guarantee electrical functioning, the electric safety, as well as the reliability of the multi-layer printed circuit board product, these bores must lie within specified tolerances relative to the internally disposed or located terminal areas (e.g. eyelets) on the signal layers as well as relative to the internally disposed etching free areas on the potential layers.
Such tolerance demands are necessary since, in the fabrication process, the printed circuit boards printed with the pattern elements are drilled at the intersection points of the grid field, whereby, for this purpose, most frequently numerically controlled drilling machines, for example, multi-spindle drilling machines, are employed. In relation to the drilling spindles of these drilling machines the printed circuit board is moved by grid steps and drilled. The bores are to be positioned precisely in the center of the eyelets, for example. It is not permissible to fall below specific minimum distances between the bore wall and the free etching border. Moreover, the bore border cannot be permitted to leave the terminal area (e.g. eyelet).
Decisive for the position accuracy of the created bores is the more or less great correspondence of the position of the zero- or reference-point of the drilling machine--operating according to a predetermined program--within a coordinate system, and hence of the individual grid steps of the bore table with the position of the reference point in the specified nominal grid field.
Very narrow ranges are to be demanded, particularly in the case of multi-layer printed circuit boards, since a precise alignment in the various layers is a prerequisite for their usefulness. Knowledge as precise as possible with respect to the actual position of the internally disposed terminal areas, or free etchings, respectively, to a reference point (e.g. location hole) is a prerequisite for satisfying this requirement.
Up to the present time, in order to solve this problem the multi-layer printed circuit board has been processed by use of an alignment or fitting system. The fitting or alignment system is disposed on each of the individual sheets or foils from which the multi-layer printed circuit board results. It consists of location holes which fix a reference point and a reference line from which the position determination of the conductive patterns on the individual sheets takes place. Also in the case of pressing the individual sheets, this fitting system is employed so that the conductive patterns of the individual inner layers following the pressing are disposed on a nominally known position relative to the location system. During drilling of the terminal holes their positioning like-wise takes place in relation to the fitting system, whereby, in most instances, the nominal position of the internally disposed conductive patterns relative to the fitting or alignment system is employed during positioning of the terminal holes.
Since the actual position of the internally disposed conductive patterns does not correspond to the normal position due to fabrication errors and changes in length, in this manner the allocation of the terminal holes to the internally disposed conductive patterns is associated with errors. Therefore, at times the attempt is also made to ascertain the difference between actual position and nominal position prior to the drilling directly on the work-piece (the pressed part) and to then correspondingly correct the drilling pattern. The known methods for determining the actual position (e.g. X-raying) operate with an accuracy of .+-.0.1 mm or greater. During the working of highly miniaturized conductive patterns, however, accuracies in the mm-range are required.